The present invention relates to a process for the preparation of novolak resins having a low content of metal ions.
The quality of photoresists can be improved by substantially reducing the amount of contaminating metal ions in the photoresists. Impurities included in photoresists mostly comprise metal ions, in particular ions of iron, sodium, barium, calcium, magnesium, copper and manganese. These impurities are attributable to a resin component which is present as a binder in the photosensitive compositions. The resin component preferably is a novolak resin which typically comprises condensation products of phenols or cresols with formaldehyde. The contaminating metal ions get into the resins primarily as a result of the preparation process. The free phenolic OH groups in the novolak resin promote the incorporation of metal ions by proton exchange and by complexing on the polar groups.
Due to this complex formation, only a minor purifying effect is achieved when the impurities included in the resin dissolved in an organic solvent are washed out with water. The expenditure for separating metal ions is increased by the low amount of only a few ppm of metal ions in the resin solutions used for the preparation of photoresists. To evaluate the quality of the purified products it is sufficient to determine the amount of iron and sodium present which serve as indicator metal cations for measuring the purifying effect.
Separation by volatilization such as, for example, by evaporation of the metal ions or the resin, is impracticable. Similarly, thermal purification processes cannot be used for novolak resins in a molten state, since the resins have a high viscosity and are instable chemically when they are heated relatively strongly. Only after dissolving in an organic solvent can the resins be subjected to a purifying operation. Precipitation processes are excluded because a precipitate of the sparingly soluble metal salts cannot be attained with the extremely low concentrations of metal ions present. Adsorption processes fail due to an insufficient selectivity of the adsorbents with respect to traces of metal ions in the organic medium. Prior to the present invention an adequate purification technique for the removal of metal traces from novolak resins had not been described and thus there is a need to solve this separation problem brought about by recent technological developments in the field of microelectronics.